Electro-spinning processes for manufacturing polymer fibers made of polymer solutions are already known for a long time [cf. for instance R. Dersch et al., Chemie in unserer Zeit, Vol. 39, pp. 26-35, 2005]. According to this principle, a plurality of polymers can be spun to fibers. The respective polymer is dissolved by means of a solvent, and applying a high voltage produces nano and/or microfibers with a minimum diameter of up to 50 nm.
There are already a certain number of publications dealing with spinning of thermoplastic polymers such as, for instance, polystyrene (PS), in the electrical field.
Frequently used solvents for electro-spinning are tetrahydrofuran (THF-S. Me-gelski et al., Macromolecules Vol. 35 (22), 8456-8466, 2002 and J. S. Stephens et al., Applied Spectroscopy, Vol. 55 (10), 1287-1290), dimethylformamide (DMF-S. Megelski et al., Macromolecules Vol. 35 (22), 8456-8466, 2002 and J. A. E. Senador et al., Mat. Res. Soc. Symp. Proc. Vol. 661, KK 5.9.1.-OKK5.9., 2001) and methyl ethyl ketone (MEK-H. F. Jia et al., Biotechnology Progress, Vol. 18 (5), 1027-1032, 2002.
The use of nano and/or microfibers, i.e. fibers with an average fiber diameter of 10 to 1000 nm, for coating fleece materials to be used as filter media is well known.
For example the methods described in DE 100 63 518 A1 and 101 55 448 A1 for manufacturing polystyrene nano and microfibers and/or for coating fleece media with thermoplastic polymer nano and microfibers are suited to be used for manufacturing filter media with a relatively high separation efficiency and relatively low aerodynamic resistance.
However, when using these thermoplastic polymer nano and/or microfibers as coating for fleece it becomes clear that the fibers deposited by the electro-spinning process have a relatively poor adhesion property on the carrier fleece. This applies particularly to structured (studded) media. The contact surface for nanofibers is quite small, for only the elevations are covered with it. When using such structured media, minor forces (e.g. winding or unwinding of the coated fleece) can detach the nanofibers. If the coated fleece is mechanically stressed due to pleating or similar processing methods, the nano and/or microfibers on the carrier fleece are severely damaged. This applies in particular to abrasive shear forces which can be applied, for example, by roller systems required for manufacturing.
To reduce the damage of the nano and/or microfibers to an absolute minimum several concepts with hot-melt adhesives or hot-melt adhesive layers have been described, for example in EP 818 230 B1. Water soluble and strongly hydrophilic polymers, such as polyvinyl alcohols, have a high tack, particularly at high air humidity, such as for example described in DE 299 076 99 U1.
Furthermore, EP 1 366 791 A1 discloses a method for manufacturing electrostatically spun polymer fibers with a cross-linking agent, such as for example polyvinyl alcohol or urea-formaldehyde resin. The use of the cross-linking agent increases the adhesion efficiency (the tack) of the nano and/or microfibers.
EP 1 733 776 A2 presents polyvinyl chloride and/or polyvinyl alcohol nanofibers stabilized against heat and impact power. Stabilization is reached by adding a cross-linking polymer, such as for example epoxy resins.
Finally, DE 44 02 857 A1 describes a blend consisting of two incompatible polymers spun in the electrical field. One blending component consists of a polymer that becomes tacky at room temperature, the other one consists of a polymer with a softening range above room temperature. Spinning such a blend electrostatically generates tacky nanofibers which form a relatively firm bond with the carrier fleece.
Consequently, the objective of the well-known solutions dealing with this problem is to obtain an improved adhesion of the nano and/or microfibers on the carrier fleece medium. This can, for example, be realized by applying bonding agents to a fleece. After that, the nano and/or microfibers are deposited on the fleece provided with the bonding agent. The adhesive is now responsible for bonding fleece and fibers. However, this approach has a few disadvantages:
If the viscosity of the adhesive is too low, i.e. if it is too aqueous, the adhesive spots will melt without ensuring a sufficient adhesion. But if the viscosity is too high, i.e. if it is too viscous, the adhesive spots do not have enough tack and there is no bonding of the fibers.
A great disadvantage of adhesive-based solutions according to the prior art is the frequent increase of pressure drop. The adhesives used tend to form large areas, thus clogging the pores of the filter medium. This significantly increases the aerodynamic resistance of the medium. When using hardenable systems, such as for example epoxy resins, other influencing factors such as the hardening time, the concentration of catalysts, mixing ratios and the like which represent potential sources of error have to be taken into account. When using UV curing systems there is the risk of damaging the carrier fleece by UV rays.